(1) Field of the Invention
The present invention relates to a high water content soft contact lens and more particularly to a high water content soft contact lens containing 50-70% of water and having excellent durability to chemical stain removers, high mechanical strengths and excellent shape retention.
(2) Description of the Prior Art
Hydrous soft contact lenses are largely divided into (a) low water content soft contact lenses containing 40% or less of water, typified by a homopolymer of 2-hydroxyethyl methacrylate (hereinafter referred to as HEMA) or a copolymer containing HEMA as a main component and (b) high water content soft contact lenses containing 40% or more of water, typified by a copolymer of N-vinyl-2-pyrrolidone (hereinafter referred to as NVP) as a main component or a copolymer of NVP and other monomer(s) such as HEMA, methacrylic acid (hereinafter referred to as MA), etc.
Hydrous soft contact lenses are soft because they contain water therein and, when worn, give little discomfort of having a foreign body sensation in the eye(s). Further, the hydrous soft contact lenses enable the migration or transfer of the oxygen dissolving in the lacrima through the lenses and can supply the oxygen which is required for corneal metabolism, to the cornea from the air through the lenses. Owing to the above advantages of the hydrous soft contact lenses, various hydrous soft contact lenses have gained wide acceptance in the contact lens market.
Of the hydrous soft contact lenses, low water content soft contact lenses are good in machinability in lens production, dimensional stability, durability to treatments for lens stain removal and durability to boiling sterilization, but are insufficient in oxygen supply to the cornea because of their low water content and accordingly, when worn for a long time, cause unexpected problems at times because of oxygen shortage in the cornea. In order to prevent it, there has been adopted in recent years a means of making smaller the thickness of low water content soft contact lens to increase the amount of oxygen supplied to the cornea. However, a smaller lens thickness results in, for example, reduction in mechanical strengths and dimensional stability of lens; therefore, the means of making the lens thickness smaller has a limitation.
Meanwhile, high water content soft contact lenses, since they contain a larger amount of water, have higher oxygen permeability and accordingly higher safety in oxygen supply to the cornea, and further have higher flexibility and give better feeling when worn, as compared with the low water content soft contact lenses. Generally in the hydrous soft contact lenses, their properties such as mechanical properties, optical properties, specific gravity, oxygen permeability and the like vary depending upon the water content in the lenses. As the water content increases, the oxygen permeability also increases but the mechanical properties decrease, which is liable to cause lens breakage in practical use of lens. Tensile strength is generally used for the evaluation of mechanical properties of a contact lens or a contact lens material. Besides, tensile modulus of elasticity can be used for the evaluation of deformation resistance of a contact lens or a contact lens material. Further, by measuring the stress relaxation of a contact lens or a contact lens material, there can be evaluated their shape recoverability from deformed state. (As a result of measuring the stress relaxations of various commercially available hydrous soft contact lenses, the present inventors found that there is a correlation between (a) the shape recovery rate of lens and (b) the stress relaxation of lens material, that is, a lens material of lower stress relaxation gives a lens of higher shape recovery rate.) Many attempts have hitherto been made in order to eliminate a problem of easy breakage possessed by high water content soft contact lenses. However, this "easy breakage" problem, together with a "stain deposition" problem, still remains unsolved in lens materials containing 70% or more of water, which allows the resulting lenses to have a short life. Meanwhile, said lens materials containing 50-70% of water and yet having a sufficient strength in practical use are already available because their development is easier than the development of lens materials containing 70% or more of water; however, these lens materials containing 50-70% of water and yet having a sufficient strength, when made into a hydrous soft contact lens, lack in flexibility, have little softness, and accordingly give discomfort when worn. In general, these lens materials have a high tensile strength and a high tensile modulus of elasticity; however, when accidentally folded in two during handling, cleaning, etc., a fold remains thereon making their optical properties unstable. This phenomenon is seen when stress relaxation appears in a lens material which has been subjected to deformation; the larger the stress relaxation, the longer is the time needed for shape recovery from the deformation and accordingly the poorer is the shape recoverability. Hence, the shape recoverability of a lens can be evaluated by measuring the value of strain relaxation when the lens has been subjected to given deformation. When evaluating the mechanical properties of hydrous soft contact lenses, their tensile strength, elongation at break, tensile modulus of elasticity and sress relaxation must fall within the given ranges required in practical application.
As another problem caused by increasing the water content, there is staining of lens. Stains of hydrous soft contact lenses include, for example, those formed by deposition of contaminants present in the lacrima or on hands, fingers, etc., on lens surface and their subsequent property change and sticking to lens surface, as well as those formed by penetration of said contaminants into lens interior. Staining of lens occurs more easily as the water content of lens is higher, and it is taken up as a big problem. That is, staining of hydrous soft contact lenses invites reduction in lens transparency and elasticity, high bacterial growth and shorter lens life. As a measure for preventing these problems, there have been adopted various methods for removing contaminants which have deposited on or penetrated into hydrous contact lenses. Accordingly it is necessary that hydrous soft contact lenses have excellent durability to stain removal treatments applied thereto.
High water content soft contact lenses comprise a copolymer comprising, as a main component, a highly hydrophilic monomer and thereby can contain a large amount of water. Various high water content soft contact lenses each having different properties can be produced by appropriately selecting the type(s) of the comonomer(s) to be copolymerized with the highly hydrophilic monomer.
Japanese Patent Application Kokai (Laid-Open) Nos. 144521/1982 and 28718/1983 disclose high water content soft contact lenses comprising a copolymer containing, as monomer components, NVP, dimethylacrylamide (hereinafter referred to as DMAA) and other monomer(s) in combination. These lenses have a certain degree of practical usability as a high water content soft contact lens, when viewed from the dimensional stability, machinability and strength in practical use. However, they have the following problems. The hydrous soft contact lens disclosed in Japanese Patent Application Kokai (Laid-Open) No. 144521/1982 uses, together with NVP and DMAA, HEMA as a monomer of largest amount, making small the total amount of NVP and an alkyl (meth)acrylate which is used as the fourth monomer (said alkyl (meth)acrylate being hereinafter referred to as RMA); as a result, the lens has low mechanical strengths and low durability to chemical stain removers (e.g. chlorine type oxidizing agent). The hydrous soft contact lens disclosed in Japanese Patent Application Kokai (Laid-Open) No. 28718/1983 contains too much the total amount (70% or more) of NVP and DMAA or too much the amount (60% or more) of NVP; as a result, the lens properties are affected too much by the combined effect of NVP and DMAA, and moreover the amount of RMA which is used together with NVP and DMAA is reduced. Consequently, the hydrous soft contact lens disclosed in the above patent document is inferior in durability to chemical stain removers (e.g. chlorine type oxidizing agent).
In general, high water content soft contact lenses, as compared with low water content soft contact lenses, have therein a larger portion (volume) occupied by water and contain a larger amount of water and accordingly easily invite deposition on lens surface or penetration into lens interior, of contaminants from the lacrima, hands, fingers, etc. As a result, the high water content soft contact lenses are stained easily, thus making it necessary and indispensable for lens users to effect stain removal of lens. Methods for stain removal can be largely divided into the following two. One is a method by mechanical stain removal wherein the surface of lens is rubbed with fingers, a specially prepared sponge or the like; the other is a method by chemical stain removal wherein a lens is treated with a surfactant-containing cleaner, an enzyme-containing cleaner, an aqueous solution containing hydrogen peroxide or a halogen type oxidizing agent, or the like. Therefore, hydrous soft contact lenses must have sufficient durability to these mechanical stain removals and chemical stain removals. As a general tendency, in high water content soft contact lenses, the higher the water content, the lower are the strength and the durability to chemical stain removers. The aqueous solution of an oxidizing agent, used for chemical stain removal has a high stain removal power and accordingly is useful, but causes side effects such as irreversible change in lens base curve, lens size, etc., and lens deformation. Any of the high water content soft contact lenses including those disclosed in the above mentioned patent documents has no durability to chlorine type oxidizing agents having a very high stain removal power.
High water content soft contact lenses, when viewed from their capability in practical use, must satisfy the requirements for durability to chemical stain removers as well as for mechanical properties represented by lens strengths and shape retention. Specifically, the high water content soft contact lenses must satisfy the following requirements (a), (b) and (c).
(a) Have excellent durability to chemical treatments for stain removal. PA0 (b) Have a tensile strength of 200 g/mm.sup.2 or more in order to provide a practical lens strength. PA0 (c) Have a tensile modulus of elasticity of 90 g/mm.sup.2 or more and a stress relaxation of 20% or less in order to have a shape retainability required to quickly recover from the relatively large deformation of lens caused by, for example, handling by hands, fingers, etc. or from the very slight deformation of lens caused by blinking and thereby to stably obtain a corrected eyesight. PA0 (A) having striking durability to chemical treatments for stain removal and, in particular, excellent durability to chlorine type oxidizing agents, PA0 (B) having, as a mechanical strength, a tensile strength of 200 g/mm.sup.2 or more and being resistant to breakage during handling, and PA0 (C) having a tensile modulus of elasticity of 90 g/mm.sup.2 or more and a stress relaxation of 20% or less and having excellent shape retainability.
A high water content soft contact lens is regarded to have no problem in practical use if it gives a base curve change of .+-.0.15 mm or less and a size change of .+-.0.20 mm or less when measured in a physiological saline solution before and after the treatment for stain removal. (According to experiences, with those lenses giving dimensional changes within the above ranges, users rarely feel discomfort of wearing.) The chemical treatment for stain removal includes, for example, a treatment by an aqueous solution of a chlorine type oxidizing agent, a treatment by an aqueous solution of an oxygen type oxidizing agent, a treatment by an enzyme-containing cleaner and a treatment by a surfactant-containing cleaner. High water content soft contact lenses must be stable to these treatments and further to boiling sterilization in water which is effected in combination therewith.
However, any prior art has been unable to provide a high water content soft contact lens satisfying all of the above requirements (a), (b) and (c).
As mentioned above, conventional high water content soft contact lenses have had serious drawbacks in practical use. While lens staining is presently unavoidable with high water content soft contact lenses, any of conventional high water content soft contact lenses has had no satisfactory durability particularly to chlorine type oxidizing agents having the highest stain removal power among various chemical stain removers.